In the recent semiconductor industry, as seen in the case of DRAM (Dynamic Random Access Memory) production,. while there has been competition for acquisition of market-share due to cost reduction of semiconductor devices, revision of the production system, for cost reduction, as well as development of semiconductor wafers used for production of semiconductor devices into large sizes (300 mm or greater) have been in progress.
As to the revision of the production system, there is an example where the system in which the whole semiconductor manufacturing plant is kept as a highly clean environment (e.g., class 10 clean or above) to manufacture semiconductor devices is transferred to the system in which multiples processing steps of semiconductor wafers are divided into sections, each interior space being maintained to be highly clean while semiconductor wafers. are conveyed by substrate storage containers between the processing steps.
According to such a new system, the installation cost for clean room construction as well as the running cost for its maintenance can be sharply cut down, and yet with increased production yield in the semiconductor device manufacturing.
The standards of the substrate storage containers used in the semiconductor production plant using this new system are specified by SEMI standards (E19, E47.1, E62, E63 and others), so that the container has a conveyance device for automated in-plant transportation and a door in compliance with FIMS, which can be attached and removed by automation. Further, the substrate storage container is demanded to have high hermetic sealing property for avoiding contamination of the semiconductor wafers and also, in order to keep the interior of the container body clean it should be formed using clean material that emits a lower amount of volatile gases.
The minimum conductor width of electronic circuits formed on semiconductor devices tends to be smaller and smaller (0.01 μm or below). In order to facilitate this, it is necessary to prevent the surfaces of the semiconductor wafers held in the substrate storage container from being formed with a natural oxide film and being contaminated with organic matter. As a countermeasure against this, there is a method called the gas purge (replacement) technique, which drives out the interior of the substrate storage container with an inert gas such as nitrogen or dry air with moisture removed (equal to or below 1%).
With regard to this, some techniques have been proposed: a filter is attached by providing a partial opening in the substrate storage container; or valves such as check valves, etc., are provided at the periphery of the opening so as to achieve gas purging (see Japanese Patent Application Laid-open Hei 11 No.191587 and Japanese Patent Application Disclosure 2002-510150).
The conventional substrate storage containers, which are constructed and gas purged as above, suffer from the following problems.
First, in the case where gas purge is performed with a valve attached to the substrate storage container, an engagement structure for valve attachment is needed. However, this engagement structure increases the complexity of the arrangement. An ordinary valve has a hollow portion with a narrow opening, and a shutoff valve that closes this opening is provided to come into contact with a tapered portion of the opening, creating a sealing structure. This shutoff valve is readily displaced by repeated usage, resultantly presenting a leakage problem due to imperfect seal. Further, since the ordinary valve incorporates more than a few metallic parts such as a spring for valve control and the like, there is a fear that a very small quantity of metal ions, discharged from the metal parts while the substrate storage container is put in storage or during its cleaning, infiltrates into the substrate storage container, contaminating semiconductor wafers.
On the other hand, in the case where a filter is merely attached to the substrate storage container, gas purge can be done with a simple configuration. In this case, however, there is a problem in that the purging gas charged through the filter, however, cannot be kept inside the substrate storage container. Therefore, the gas in the substrate storage container can easily leak out, so that the gas purge effect can be maintained in only a short period. There is also a problem in that it is impossible to effectively prevent a trace amount of organic matter in the clean room from flowing into the interior of the substrate storage container from exterior because there is a passage between inside and outside of the substrate storage container.